1. Field of the Invention
The present invention relates to a method of cutting a stack structure and the stack structure, and in particular to a method of cutting a stack structure of different materials with a laser beam.
2. Description of the Related Art
State-of-the-art devices such as a MEMS (microelectro mechanical system) device has a structure in which a great variety of materials such as LiTaO3, sapphire and heat-resistant glass as well as silicon are stacked in layers. The MEMS device including a sensor such as an acceleration sensor or a mechanical switch has a fine mechanism. A multiplicity of MEMS devices are normally formed on a silicon wafer, which is sandwiched by two glass plates to seal the MEMS devices in vacuum. After that, a MEMS device is cut from the stack of the silicon wafer and the glasses.
Generally, a multiplicity of MEMS devices formed on a silicon wafer substrate are cut into chips by rotating a disk-shaped wheel having a blade containing diamond abrasive grains at high speed. However, in the case where MEMS devices having a fine mechanism are cut into a multiplicity of chips by this method, a large amount of vibration is caused during the cutting process, thereby often breaking the fine mechanism of the devices.
A method of causing no vibration of these devices having a fine mechanism consists in cutting the wafer in noncontact way. In the prior art, a method has been proposed in which two laser oscillators are arranged for a double-layer stack structure to use a laser of a wavelength corresponding to the material of each layer to be removed (Japanese Unexamined Patent Publication No. 2003-37218). Similarly, a method has been proposed in which a stack structure of multiple layers each having a different wavelength absorption band is cut using as many laser light sources as the layers (Japanese Unexamined Patent Publication No. 2004-181463). In both of these methods, however, a plurality of laser oscillators are required, resulting in a bulky and expensive system.
The glass plate sealing the MEMS devices transmits visible light and therefore cannot be cut without using an expensive laser beam machine such as a far infrared laser or an ultraviolet laser. Further, the far infrared laser cannot be converged into a thin beam and may burn the devices, while the ultraviolet laser requires a very long time to cut the wafer.
The present inventor et al. have proposed a method of cutting a double-layer stack structure using a single laser (Japanese Patent Application No. 2004-168312). According to the method, grooves are formed in one of first and second substrates, the grooved substrate is stacked on the other substrate with the grooved-surface facing the other substrate, and a laser is emitted to the grooved-substrate along the grooves to cut the stack structure including the first and second substrates. This method, however, is not adapted to cut a stack structure having three or more layers.